1. Field of the Invention
This invention generally relates to continuous tank-type glass melting furnaces and, more particularly, to such furnaces having waist areas of reduced cross section interconnecting the melting and working zones whereat the flow, temperature and homogeneity of a glass bath may be controlled.
2. Description of the Prior Art
Conventionally, one type of continuous glass melting furnace is constructed with a lower tank section covered by an independently supported roof section, wherein the central region of the tank section is provided with a reduced waist which provides a passage of limited cross section intermediate the ends of the tank. This construction, in effect, forms separate melting and conditioning or working tanks or zones interconnected by a passage that is slightly smaller in width than the tanks so that the melting operation can be performed to optimize conditions in the melting tank and deliver properly refined molten glass to the conditioning zone. The size of the waist and, of course, the passage, is determined by the output and the operating conditions of the melting furnace. Such tanks are generally, but not necessarily, of constant depth and their width may or may not be constant, except for the waist section where the side walls are inset to provide the narrow region interconnecting the refining and conditioning zones of the tank for improving the homogeneity of the molten glass as it passes from the refining zone into the conditioning zone.
In such a continuous tank-type glass melting furnace, raw batch material and scrap glass or cullet are charged into one end of the furnace and molten glass is removed from its other end. The glass, in moving through the furnace, passes successively through melting, refining, conditioning or cooling, and working zones which are contiguous with one another. Heat is applied over the upper surface of the bath of glass in the melting zone through ports along the sides for reducing the newly added materials to a molten state and integrating them into the flowing molten bath, and the molten glass bath is refined and cooled to a point where it can be removed from the working zone to form a continuous ribbon.
Addition of heat to the molten glass bath and the charging of relatively cold glass making materials establish varying temperatures in the bath throughout the length of the tank. These variant temperatures, with other processes occurring in the tank, result in formation of a zone of maximum temperature commonly called a "hot spot". This hot spot normally occurs slightly downstream from the midpoint of the port area. One result of the temperature differential and the resulting hot spot is that thermal or convection currents are established in the molten bath which are of such directions that, rearwardly of the hot spot, the upper region of the molten glass tends to flow towards the charging end of the tank and forwardly of the hot spot, the upper region of the glass tends to move towards the discharge end of the tank. These convection currents are useful in that they tend to, in effect, create a barrier between the melting and fining zones beyond which any unmelted batch materials on the surface do not pass, as well as intermix the molten glass within the respective zones.
Since the instant invention is concerned with that circuit of the convection currents flowing from the hot spot towards the discharge end of the furnace, the following discussion will be limited thereto. This convection current is composed of two thermal currents, i.e, an upper, forwardly flowing surface current and a lower, rearwardly flowing return current. As is known, the surface of the molten bath is formed by hot glass of relatively low density, and it moves from the melting zone through the refining zone towards the discharge end of the tank where only a portion of the molten surface glass is removed from the furnace. As the remaining surface glass cools, its density increases and the cooler, more dense molten glass sinks to the lower levels of the bath to join the rearwardly flowing return current which flows back toward the hot spot of the furnace. In the melting zone of the tank, the molten glass in the return current becomes heated, rises to the surface at the hot spot, and begins to circulate again in the surface current.
As the molten glass advances along the tank from the melting zone, there may be areas of non-homogenous composition as well as temperature variations in the molten bath. As the molten glass bath passes through the refining zone, where a certain amount of mixing occurs due to the convection currents, entrapped gases are released and the molten bath becomes more equalized in temperature and composition. Since the surface of the molten bath may have a higher temperature than the remainder of the bath, there is a tendency for the surface layer of glass to flow more quickly through the waist area and into the working zone. To control the flow of the surface layer and cause intermingling of the glass in the upper region, floaters and skimmers, such as disclosed in U.S. Pat. No. 3,989,497, issused to Dickenson et al on Nov. 2, 1976, have been provided at the entrance end of the waist region. Also as shown therein, stirrers have been provided to further improve the homogeneity of the molten bath. Although these devices have served their intended purpose, still further improvement is sought in the homogeneity of the molten glass and utilization of the heat provided to the melting furnace.
Accordingly, it is desirable to improve the homogeneity of the molten bath to eliminate and/or reduce the number of optical defects that may occur in a glass ribbon produced from the molten bath, and to modify the temperature pattern within the molten glass in order to better utilize the heat supplied to the tank.